1. Field of the Invention.
This invention relates to power supply systems for feeding inductive elements and in particular to power supply systems for feeding switched inductive windings such as the phase windings of a switched variable reluctance motor. A motor of this kind, to which the present invention may be applied, is disclosed in our co-pending Patent Application Ser. No. 798,038 entitled "Variable Speed Variable Reluctance Electrical Machines".
The invention also relates to power systems in general, and in particular to certain configurations of power systems serving as dc (direct current) to dc converters.
2. Description of the Prior Art.
In a variable reluctance motor provided with a unipolar drive, current may be switched into the phase windings by electronic devices under PWM control. In considering the electrical behaviour of the power supply circuit for the phase windings, the windings may be regarded as inductors under certain operating conditions, in that their response to current flowing through them under such circumstances is largely determined by their inductance rather than by their resistance. When the switch for a particular winding is closed, current flows through the inductor in question, which may then be connected between a supply rail and ground. Energy is thus stored in the magnetic field of the winding, the amount of this energy being 1/2i.sup.2 L, where i is the current and L is the inductance of the winding. When the switch opens again, this energy stored in the magnetic field has either to be dissipated or, preferably, returned regeneratively to the supply. In a particular supply arrangement for reluctance motors, the switched end of the winding is also connected to a second supply rail through a normally reverse biassed diode. Thus in this arrangement, winding current transfers to the diode after the switch has opened and decays if the second supply rail has the appropriate polarity. However it is frequently the case that such supply rails are unable to regenerate energy, with the possible result that the voltage of the second supply rail may rise to a destructive level unless an equal or greater current is drawn from it than that being supplied by the phase winding. Dealing with or disposing of this inductively stored energy is thus a considerable problem in the application of variable reluctance motors, especially in situations where they are required to operate at low rotational speeds.
A twin-rail power supply with an equal number of motor phases connected to the positive and negative rails may be adequate for reluctance motors operating in a continuously-rotating motoring mode only, with non-usable inductively stored winding energy being returned to the complementary rail for use in the phases connected thereto, but this is not necessarily the case in a motor required to provide torque at zero speed in order to hold a load, where the current of the driving phases, less losses, may be transferred continuously between the rails and may pump up a supply capacitor located, for example, between the second rail and earth. At certain rotor positions the effects of two phases will cancel, and between these points, peaks of upward and downward current transfer will be reached. Thus, the effect at zero speed is to unbalance the rails. On the other hand, in a reluctance motor rotating at speed and acting to decelerate an inertial load or otherwise regenerate energy, the effect will be to pump up both supplies.
This second-mentioned effect is the same as that which exists in any conventional servodrive, and since the total energy involved in a typical duty cycle is not great, it may be dealt with by burning it off in a dump resistor disposed between the rails. As a rule of thumb, the dump resistor is usually sized to intermittently draw a current equal to the continuous rating of one axis in a DC drive (e.g. 20A or 40A). The first effect, i.e. that at zero speed, is not seen in DC servodrives with a single rail supply. One solution to the problem is to switch the reluctance motor phases at both top and bottom but this doubles the number of main devices.
A second technique to regenerate phase energy into the main supply is to use special bifilar windings in the motor. While this may seem attractive from many points of view there are also serious problems with this approach, as noted below, since the number of connections to the motor is doubled. In particular, to allow for worst case duties, the secondary winding would need to have virtually the same cross-section as the primary, thus greatly reducing the utilization of winding area and motor rating. In addition, in a bifilar winding, two closely coupled coils are connected to opposite supply rails and may have very high potential differences between them, leading to unreliable operation and breakdown. While appropriate for low voltage battery operation, this could cause serious problems with supplies over 100 V. Also as a main transistor switches off and a secondary winding takes over current conduction, very fast current rises and falls would take place in the leads to the winding. This, along with poor coupling between primary and secondary windings, could give rise to severe electromagnetic noise radiation. Bifilar windings may thus be seen to be appropriate only when the drive electronics are mounted close to the motor.
Since in virtually every application a servomotor requires to hold the friction torque of the mechanism it is driving when at stall, and stall current can be up to half the motor continuous rating, and since also in many applications, the motor will be holding an uncounterbalanced load, the provision of some effective and economical means of transferring energy away from a supply undergoing pump-up is regarded as a necessary feature of at least servomotor drives.
It may be argued that in large multiaxis systems, conditions at large should cancel out, so as to make the problem a relatively minor one. On the other hand, a solution to the unbalance problem must be available for implementation in systems where it is required. A very crude solution would be to have individual dump resistors on the rails to burn off the unbalance. However, since this might involve burning off the rated motor current continuously, it would hardly be acceptable.